1. Field of the Invention
The present invention relates generally to an apparatus of measuring the optical characteristics of the boundary area of a given medium to be inspected.
2. Related Background Art
The following conventional methods are known, which measure various optical characteristics by using total internal reflection of light.
(1) TIR Method (a total internal reflection method, J. Z, Xue, N. A. Clark, and M. R. Meadows, Appl. Phys. Lett. 53, p. 2397 (1988)).
(2) Calibration Curve Method (JP-A-6-82239)
As an example of an optical-characteristic measuring apparatus that utilizes each of the above methods, an optical anisotropy measuring apparatus for measuring a pre-tilt angle of liquid crystal will be described.
(1)-1 Optical anisotropy measuring apparatus utilizing the TIR method (Prior Art 1)
FIG. 1 is a schematic diagram showing an example of the structure of an optical anisotropy measuring apparatus utilizing the TIR method and illustrating an optical anisotropy measuring method. This optical anisotropy measuring apparatus 1 has a glass member of a semispherical shape (hereinafter called a "semispherical glass"). As shown in detail in FIG. 2, this semispherical glass 2 has a flat part 2a and a spherical part 2b. A glass substrate 3 is disposed facing the flat part 2a. On the surfaces of the glass substrate 3 and flat part 2a, transparent electrodes 5 and orientation films 6 are formed. The glass substrate 3 and flat part 2a are bonded together by a sealing member 7. Liquid crystal 9 as a medium to be inspected is filled in a space between the glass substrate 3 and flat part 2a.
The semispherical glass 2 is supported by an unrepresented rotation mechanism to rotate the spherical glass 2 about a rotary axis C normal to the flat part 2a. The refractive index of the semispherical glass 2 is set larger than that of the polarization film 6 and liquid crystals 9. The film thickness of the orientation film 6 is made smaller than the wavelength of an applied light beam A1 (the details will be given later).
An He--Ne laser source 10 is disposed on one side (lower left in FIG. 1) of the semispherical glass 2 and applies the light beam A1 to the semispherical glass 2 along a downward oblique direction relative to the flat part 2a (the light beam A1 applied to the liquid crystal 9 from the He--Ne laser source 10 is called hereinafter an "incidence light beam A1"). A photodetector 11 is disposed on the other side (lower right in FIG. 1) of the semispherical glass 2 and detects a light beam B1 totally reflected from the interface to the liquid crystal 9 (the light beam B1 totally reflected is hereinafter called a "reflection light beam B1").
A polarizer 12 is disposed between the semispherical glass 2 and He-Ne laser source 10 and linearly polarizes the incidence light beam A1 from the He-Ne laser source 10. An analyzer 13 is disposed between the semispherical glass 2 and photodetector 11 and has a polarization direction perpendicular to the polarizer 12.
Next, an optical anisotropy measuring method (a pre-tilt angle measuring method) using the above-described optical anisotropy measuring apparatus will be described.
The incidence light beam A1 output from the He--Ne laser source 10 is linearly polarized by the polarizer 12 into p-polarization relative to the incidence plane of total internal reflection, and applied to the semispherical glass 2. This incidence light beam A1 is totally reflected by an interface between the transparent electrode 5 and orientation film 6. Evanescent light, generated when the total reflection occurs, enters once the liquid crystal and then is reflected. This evanescent light changes its polarization state in accordance with the optical anisotropy of liquid crystal near at the interface to the orientation film 6.
Of the reflection light beam B1 output from the semispherical glass 2, only the components (s-polarization components) having a polarization direction perpendicular to the polarizer 12 pass through the analyzer 11.
As the semispherical glass 2 together with the glass substrate 3 and the like is rotated about the rotary axis C, the direction of a director, which is a unit vector representative of the direction of a liquid crystal molecule of the liquid crystal 9, changes with the direction of an electric field of the incidence light beam A1. Therefore, the polarization state of the reflection light beam B1 output from the semispherical glass 2 changes with the rotational angle of the semispherical glass 2. By plotting an output of the photodetector 11 relative to the rotational angle of the semispherical glass 2, a characteristic curve representative of the optical anisotropy of the liquid crystal, such as shown in FIG. 3, can be obtained. The pre-tilt angle can be calculated from a ratio of Imax/Imin, where Imax is a maximum extreme intensity and Imin is a minimum extreme intensity. The larger the pre-tilt angle, the smaller the ratio of Imax/Imin becomes, whereas the smaller the pre-tilt angle, the larger the ratio of Imax/Imin becomes.
With the optical anisotropy measuring apparatus 1 described above, the optical anisotropy or pre-tilt angle of the liquid crystal 9 is calculated in accordance with a change in the polarization state of the reflection light beam B1 to be caused by the interaction between the liquid crystal molecules and the evanescent light generated when the total reflection occurs.
The measurement area (an ellipsoid having a minor axis of about 0.6 mm and a major axis of about 3 mm) of the optical anisotropy measuring apparatus 1 is larger than the size (a square of 30 to 50 .mu.m) of one pixel of a liquid crystal device used with a display or the like. Therefore, the orientation state of each unit pixel cannot be measured so that the orientation states of pixels cannot be compared. It is also difficult to detect a variation in orientation directions of one pixel. It is also impossible to detect a fine defect smaller than, for example, 8 .mu.m and it is difficult to compare the orientation state of a defect area with that of another area. From the above reasons, the orientation state of the liquid crystal 9 cannot be detected correctly and it is difficult to elucidate the mechanism of defect formation.
(1)-2 Optical anisotropy measuring apparatus utilizing the TIR method (Prior Art 2).
To solve the above problems, an apparatus 20 shown in FIG. 4 has been proposed (JP-A-9-105704) which has an input side optical system 31 disposed between an He--Ne laser source 10 and a semispherical glass 2 to converge an incidence light beam A1 and make the measurement area small (a major axis of about 10 to 30 .mu.m). In FIG. 4, reference numeral 22 represents a liquid crystal device, and reference symbol A2 represents an incidence light beam converged by the input side optical system 31. The major axis of the measurement area of this apparatus 20 is about 8 .mu.m.
(1)-3 An optical anisotropy measuring apparatus utilizing the TIR method (Prior Art 3).
In the Prior Art 1, the orientation film 6 is formed directly on the side of the semispherical glass 2. However, it is very difficult to form good medium samples by subjecting the orientation film 6 to a rubbing process.
To form a good medium, an apparatus has been proposed (JP-A-9-105704) that uses a discrete liquid crystal device and a discrete semispherical glass 2. With this apparatus, since the liquid crystal device is movable relative to the semispherical glass 2, a variation of pre-tilt angles can be measured (refer to Technical Digest, the Fifth Microoptics Conference (MOC' 95 Hiroshima), G10, p. 144, by Y. Ohsaki and T. Suzuki).
(1)-4 An optical anisotropy measuring apparatus utilizing the TIR method (Prior Art 4).
The major axis of the measurement area of Prior Art 2 is about 8 .mu.m. An optical anisotropy measuring apparatus 30, such as shown in FIG. 5, has been proposed (in JP-A-9-148283) which makes the major axis smaller.
This optical anisotropy measuring apparatus 30 has two upper and lower semispherical glasses 2 (hereinafter called an "upper semispherical glass 2" and a "lower semispherical glass 2" when discrimination therebetween is necessary). Each of the semispherical glasses 2 has a flat part 2a and a spherical part 2b. The semispherical glasses 2 are disposed facing each other at a predetermined distance between the flat parts 2a and 2a.
A liquid crystal device 22 is disposed between the semispherical glasses 2. As detailed in FIG. 6, the liquid crystal device 22 has a pair of glass substrates, and a transparent electrode 5 and an orientation film 6 are formed on the surface of each of the glass substrate 23. The glass substrates 23 are bonded together by a sealing member 7. Liquid crystal 9, as a medium to be inspected, is filled in between the orientation films 6.
Refractive index matching liquid 25 is filled in between the liquid crystal device 22 and each semispherical glass 2, the liquid having generally the same refractive index as the glass substrate 23 and semispherical glass 2. Therefore, reflection does not occur at the interface between the liquid crystal device 22 and each semispherical glass.
An input-side optical system 31 is disposed between a polarizer 12 and the lower semispherical glass 2. An He--Ne laser source 10, a polarizer 12, and an input side optical system 31 are disposed so that a partial light beam A3 becomes incident at an angle smaller than a critical angle .theta.c. A fraction of the light beam B3 transmits through liquid crystal 9, the light beam transmitting through the liquid crystal 9 being called a "transmission light beam B3".
On the opposite side of the lower semispherical glass 2, an output side optical system (second optical system) 32, an analyzer 13, and a photodetector 11 are disposed so that a reflection light beam B2, totally reflected at the liquid crystal interface, is detected with the photodetector 11, which measures optical anisotropy to detect the orientation state of the liquid crystal interface.
Also with this apparatus 30, the pre-tilt angle can be calculated by the method that is the same as in Prior Art 1.
In this case, however, the light beam to be measured is the light beam B2 incident upon the liquid crystal at an angle larger than the critical angle .theta.c and totally reflected at the liquid crystal interface. The partial light beam A3 is incident upon the liquid crystal at an angle smaller than the critical angle .theta.c so that it is not totally reflected, but most of the partial light beam is transmitted through the liquid crystal 9 and becomes the transmission light beam B3, and the remaining light beam becomes an ordinary reflection light beam. This ordinary reflection light beam has an unchanged polarization state different from the totally reflected evanescent light, and cannot transmit through the analyzer and cannot be detected with the photodetector 11. The spherical area 2b of the upper semispherical glass 2 disposed on the liquid crystal device 22, with refractive index liquid 25 being interposed therebetween, is formed with an antireflection film. Therefore, the transmission light beam B3 is neither reflected at the interface to the antireflection film nor detected with the photodetector 11.
With this apparatus 30, since the partial light beam A3 is made incident at an angle smaller than the critical angle .theta.c, the numerical aperture NA becomes large and the light beam A2 incident upon the liquid crystal 9 is more strongly converged (specifically being able to be converged to a light flux diameter of 2 .mu.m or smaller) so that the measurement area can be made about 5 .mu.m.phi. or less. Therefore, the light beam illumination area on the liquid crystal interface becomes circular from a conventional ellipsoidal shape.
Under such coherent illumination, the diameter (measurement area) of a focal point is given by: EQU 1.4.lambda.(N.sup.2 +0.25/n+L )
where N is a f-number of a lens of the input side optical system 31, .lambda. is a wavelength of the incidence light beam A2, and n is the refractive index of the glass substrate 23. If the refractive index of the liquid crystal 9 is 1.5 and the refractive index of the semispherical glass 2 and glass substrate 23 is 1.8, then the total reflection critical angle Oc is represented by sin .theta.c=1.5/1.8, i.e., the critical angle .theta.c is 56.4.degree.. If .lambda.=0.63 .mu.m and the incidence angle .theta.=45.degree., the diameter (measurement area) of a focal point is about 0.8 .mu.m at N=1 and about 1.1 .mu.m at N=1.5.
Since the measurement area is made considerably small as compared to the size of one pixel of the liquid crystal, the distribution of pre-tilt angles in one pixel can be measured and the orientation states of the liquid crystal 9 in a fine defect area and in a nearby area can be measured. Therefore, this apparatus 30 is a very effective means not only for the development of the liquid crystal 9 and orientation film but for the development of orientating methods themselves.
(2)-1 An optical anisotropy measuring apparatus utilizing the calibration curve method (Prior Art 5).
This apparatus has a similar structure to that of the apparatus of Prior Art 1 (FIG. 1) because total reflection is utilized. However, a different point is that it is not necessary to rotate a semispherical glass 2. An analyzer 13 is also unnecessary. Incidence light beams A2 of p-polarization and s-polarization are used and the reflectivities of total reflection light beams are measured and a logarithmic ratio of these reflectivities (two-color ratio of light absorption) is calculated. This ratio is used as a parameter in searching a calibration curve representative of a relation between a pre-tilt angle and a two-color ratio obtained beforehand by another pre-tilt angle measuring method to thereby obtain the pre-tilt angle. Therefore, not the He--Ne laser source 10 but an infrared light source or an ultraviolet light source is used in correspondence with the absorption spectrum of the liquid crystal. The material of the semispherical glass 2 changes with the wavelength of light to be used, for example, silicon, germanium or the like if infrared light is used, and sapphire if ultraviolet light is used.
The optical anisotropy measuring apparatus 30 of Prior Art 4 has advantages of a fine measurement area and the like, which Prior Arts 1 to 3 do not provide. On the other hand, since the semispherical glass 2 is disposed also above the liquid crystal device 22, the position of the measurement area is hard to be visually confirmed from the position above the liquid crystal device 22.
The position of the measurement area may be checked by removing the upper semispherical glass 2 and thereafter the pre-tilt angle is measured by mounting the upper semispherical glass 2. However, this method complicates the measurement work, and in addition there is a possibility that a position displacement may occur while the upper semispherical glass 2 is mounted. If the f-number N is small as in Prior Art 4, the size of the measurement area changes greatly and the measurement precision lowers if the focal point shifts even slightly in an optical axis direction.
The illumination area (measurement area) may be observed through the upper semispherical glass 2 with a microscope objective lens having a long work distance, without dismounting the upper semispherical glass 2. With this method, however, there are restrictions in the size of the semispherical glass 2 and the magnification factor and resolution of a microscope objective lens. In order to eliminate these restrictions, the apparatus may become expensive and the measurement work performance may be degraded.